55Fe cytocide of the differentiated visible erythroid compartment is used to study the feedback regulation of rate of differentiation of pluripotent stem cells into the erythroid pathway. A spontaneous mammary carcinoma of the mouse which produces a granulocytosis has been maintained in tissue culture. It produces colony stimulating factor (CSF) in culture. Transplantation of this cultured tumor in graded numbers of cells is used to study the mechanism by which the tumor produces a granulocytosis. An antibody to CSF, produced by Richard Shadduck suppresses granulocyte production in normal mice. In mice with a transplanted tumor there is no detectible effect as yet.